Many systems are known, which involve gas/liquid contact, e.g. for the aerobic digestion of biomass, using oxygen or air, and for the disinfection of water, using chlorine or, increasingly ozone. These systems are generally based on the principle of introducing bubbles of the gas into the liquid, e.g. in counter-current, with a view to maximising the degree of contact between the active gas and the liquid.
A known water disinfection tank comprises an inlet and an outlet for liquid and defines a flow plath including, in succession, a downward flow or diffuser zone including a gas bubble inlet (e.g. for chlorine or ozone gas) and an upward flow or decay zone, the zones being separated by a wall beneath which there is a gap for the flow between these zones. The final decay zone is a relatively large chamber, in which it is intended that all residual disinfectant should stay in contact with the liquid for a specified time, before passing to the outlet of the apparatus. Prolonged contact is necessary, in order effectively to kill organisms such as Giardia.